In short, a manta ray is a type of stingray. A manta ray is to a stingray like a boa is to a snake.
I've noticed some people will slap term "manta ray" onto any old stingray, which is incorrect, as a manta ray is a specific genus within the ray group.
This is what a manta ray looks like:
|The manta ray has distinct white markings and cephalic fins on either side of the head. And it's huge.|
|A southern stingray is NOT a manta ray.|
|A cownose ray is NOT a manta ray.|
|A spotted eagle ray is NOT a manta ray.|
|A manta ray IS a manta ray.|